It is known that high cristobalite, also referred to as beta-cristobalite, is one of the best refractories available for use in ovens and furnaces where the temperatures encountered are well-above 1,000.degree. C and up to 1,500.degree. C, due to its low coefficient of thermal expansion.
Silica brick is the refractory commercially available and it usually contains a mixture of low (alpha)-cristobalite other low temperature forms of SiO.sub.2, including quartz and tridymite, residual glass and some anorthite. When the silica brick refractory is in place as a lining material for a furnace, and is subjected to temperatures up to 1,500.degree. C, substantially all of the other forms of silica are converted over a period of time to the high cristobalite crystal phase. However, when the furnace is shut down for repairs or other reasons and the high cristobalite-containing-silica brick is cooled to ambient temperatures, an inversion of the high-cristobalite to low-critobalite occurs when the brick is cooled below 200.degree.-275.degree. C. Low cristobalite has a higher coefficient of thermal expansion than high cristobalite. This inversion to low-cristobalite produces a change in the volume and symmetry of the high cristobalite, which change results in spalling and cracking of the silica brick, rendering it unusable as a furnace lining. Such bricks must be replaced before the furnace can be fired up again.
The inversion of high (beta)-cristobalite to the low (alpha)-form upon cooling to below about 200.degree. C is recognized in U.S. Pat. No. 3,445,252 and three approaches for eliminating this undesirable inversion are discussed therein. The first is to eliminate or substantially inhibit the growth of cristobalite-type crystals by adding minor amounts of Na.sub.2 O, K.sub.2 O, Rb.sub.2 O, Cs.sub.2 O, CaO, SrO, BaO, La.sub.2 O.sub.3 and/or P.sub.2 O.sub.5 to the glass melt such that the crystallization formed was essentially all mullite (3Al.sub.2 O.sub.3 .2SiO.sub.2). The second approach is to promote the growth of low-crystobalite-type crystals upon the heat treatment of Al.sub.2 O.sub.3 -SiO.sub.2 glasses by the addition thereto of at least 0.5% but no more than about 3% by weight of Li.sub.2 O. The third approach is the addition of at least 1% but not more than 5% by weight total of CaO, CuO or SrO to the Al.sub.2 O.sub.3 -SiO.sub.2 glasses to stabilize the high-cristobalite-type present with the mullite crystals. The patentee does not disclose a highly essential feature of the compositions of the present invention, namely that the ratio of moles of CaO (or CaO plus certain other oxides, explained hereafter) to Al.sub.2 O.sub.3 is essentially 1. The highest molar ratios of CaO to Al.sub.2 O.sub.3 in the examples of Table I of the patent are 0.44 (Example 22) and 0.34 (Example 6). Moreover, U.S. Pat. No. 3,445,252 does not teach the art how to make a glass-ceramic having stabilized high-cristobalite not containing mullite or such a glass-ceramic having over 50% and particularly at least about 70% and higher high-cristobalite solid solution crystal phase. The glass-ceramics obtained by following the teaching of the patent contain a mixture of high-cristobalite or low-cristobalite with other crystal phases. The high-crystobalite glass-ceramic products described therein always contain mullite. Moreover, according to U.S. Pat. No. 3,445,252, the alkali metal oxides such as Na.sub.2 O, K.sub.2 O, Rb.sub.2 O and Cs.sub.2 O tend to inhibit the growth of the cristobalite derivatives and thus it is recommended to leave them out of the cristobalite compositions. As will be described below with respect to the present invention, CaO is present, in certain critical amounts, to produce stabilized high-crystobalite solid solutions and part of the CaO may be replaced by a metal oxide M.sub.2 O, where M is a cation of the first group of the Periodic Table, which includes Li, Na, K, Rb and Cs. Use of such CaO and M.sub.2 O produces results, i.e., stabilized high-cristobalite solid solutions, which results are contrary to those one would expect from the disclosure in U.S. Pat. No. 3,445,252.